Blinking suppression and biexcitonic emission in thick-shell CdSe/CdS nanocrystals at cryogenic temperature

The fluorescence of single colloidal thick-shell CdSe/CdS nanocrystals (NCs), at cryogenic temperature (4 K) and room temperature (RT), is studied using the intensity autocorrelation function (ACF) and lifetime measurements. The radiative and Auger decay rates corresponding to the desexcitation of the charged biexcitonic state are determined through an original method of photon postselection. Especially, the charged biexciton quantum yield increases from about 15% at RT to 60% at 4 K. The high inhibition of Auger recombination already observed for the trion state of CdSe/CdS NCs at low temperature is also demonstrated for the charged biexcitonic state. At 4 K, the ACF is equal to 1 for time scales ranging from 50 ns to 200 ms. In contrast with RT operation, the intensity of the trion emission is then perfectly stable and no blinking is observed. All the results highlight the strong confinement of the charge carriers in the CdSe core.

Figure 1

(a) Fluorescence intensity trace of a single NC at RT in air (time bin = 2 ms, dark counts = 0.4 per 2 ms). (b) Fluorescence intensity trace of a single NC at 4 K (time bin = 10 ms, dark counts = 15 per 10 ms).

Figure 2

(a) Coincidence counts at 4 K. The area of the peak at zero delay is 60% of the mean area of the lateral peaks (after background correction). (b) Coincidence counts of the emission of another NC when it is in an ionized state at RT. The area of the peak at zero delay is then 40% of the mean area of the lateral peaks (after background correction).

Figure 3

(a) PL decay corresponding to X${}^{*}$ recombination. The photons are selected from the peak at zero delay [see Fig. 2]. The red line is a monoexponential decay (with an offset, lifetime of 6.1 ns). (b) PL decay corresponding to BX${}^{*}$ recombination. The photons are selected from the peak at zero delay of Fig. 2. The red line is a monoexponential decay (with an offset, lifetime of 1.5 ns). (c) Total PL decay. The red line is a biexponential decay (with an offset, lifetime of 1.5 and 5.7 ns). (d) PL decay corresponding to X${}^{*}$ recombination at RT (the photons are postselected using a threshold intensity as in Fig. 2). The red line is a triexponential decay [with an offset, lifetimes of 1.5 ns (17%), 15 ns (68%), and 36 ns (15%)]. The first component corresponds to the BX${}^{*}$ state, and the second one corresponds to the X${}^{*}$ state. The last component corresponds to residual photons of the X state that have not been filtered. The discrepancy between the value of 36 ns and the X lifetime, greater than 60 ns, comes from the low number of photons corresponding to the recombination of the X state.

Figure 4

(a) Normalized area of the peaks of the autocorrelation function at 300 K. (b) Normalized area of the peaks of the autocorrelation function at 4 K.

Figure 5

Mandel factor $Q\left(T\right)$ at 300 K (black line) and 4 K (red line).